Stabilization of semiconductor photocathodes

ABSTRACT

A stabilization technique is provided for photocathodes if image intensifier and related opto-electronic tubes, wherein loss of the volatile element or elements in the photocathode is inhibited by providing a reservoir of each such element within the tube in order to maintain the pertaining partial pressure of said element.

United States Patent Stahl Feb. 25, 1975 Attorney, Agent, or FirmRobert P. Gibson; Nathan [75] Inventor: Herbert A. Stahl, Springfield, Va. Edelberg; John E flolford [73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC. [57] ABSTRACT [22] Filed: Oct. 12, 1973 2 Appl' N 406,047 A stabilization technique is provided for photocathodes if image intensifier and related opto-electronic t bes, he 10 of th 1 t1 1 t l t 521 US. Cl. 313/94, 313/179 the g figg gg is 5,33,11,13 gj3 g g gfggj [51] II'II. Cl. Holj 39/00 veil. of each Such element within the tube in Order to [58] held of -5137196 1 1 43? ggbl z lg zfilj :zifatain the pertaining partial pressure of said ele- [56] References Cited UNITED STATES PATENTS 8 Claims, 2 Drawing Figures 3,329,853 7/1967 Neuhauser 313/179 STABILIZATION OF SEMICONDUCTOR PHOTOCATI-IODES BACKGROUND OF THE INVENTION The development opto-electronic devices of high sensitivity and real-time capability has depended heavily so far on the crystalline semiconductor technology. The group IV elements such as silicon and germanium have played an essential role, but compounds of elements from groups III and V of the Periodic System have become equally important, if not more important, as cathode materials. All these compounds exhibit the same or a similar lattice structure. Unfortunately, these latter substances do not behave in the same way as the single element type. The compound lattices tend to decompose gradually even at room temperature owing to a slow evaporation or sublimation of the more volatile constituent. When doping elements are added to the lattice the problem becomes more complicated. Both silicon and germanium as well as IlI-V-compounds under such circumstances can be thusly effected by ag- SUMMARY OF THE INVENTION The object of this invention is to inhibit the degradation of such semiconductor photocathodes by controlling the environment in which they operate. This will provide, for example, third generation image intensifier tubes with longer life and uniform operating characteristics over its lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects of the invention are best understood with reference to the drawings wherein:

FIG. 1 shows a plot of vapor pressure versus temperature mainly for certain elements from groups III, IV and V of the Periodic System including those that are commonly used in zero affinity-type photoelectric emitters.

FIG. 2 shows a partial view of a commercially available image intensifier tube to which stabilization coatings (also called flashes or mirrors) have been applied.

DESCRIPTION OF THE INVENTION The last two curves 11 and 12 to the right in FIG. 1 illustrate that, for germanium and silicon below 800 Centigrades, the vapor pressure is extremely low. At any feasible operating temperature it is clear that cathodes made solely from these materials would not readily evaporate lattice atoms, and thus disturb the electronically optimal surface structure. In practice, however, most such cathodes employ cesium as the sensitizing agent proper of the material. For this reason, the vapor pressure curve 13 of this metal is also entered into the graph, although cesium itself is not covered in the present disclosure. (Upon sensibilization, such devices are routinely flooded with cesium vapor so that the topic of this disclosure does not apply). The vapor pressure curve 14 of mercury is also shown for comparison only.

Vapor pressure curves akin to those of silicon and germanium are typical for indium, gallium and aluminum l5, l6, 17, respectively, which represent the electronegative constituents of most already conventional, zero-affinity photocathode materials. Finally, the solid curves 18, 19, and 20 illustrate the vapor pressures of phosphorus, arsenic and antimony, respectively, which are the electropositive constituents of the aforementioned III-V, zero-affinity photocathode materials.

It ought to be mentioned that extreme cleanliness is of paramount importance in all sorts of optoelectronic devices. Bakeout, and best vacua are routinely employed to remove all evaporable contaminants from within the envelope. This leaves in the completed device a host of internal surfaces which tend to trap any volatilizable material which may have subsequently been desorbed from the photoelectric cathode.

FIG. 2 shows a portion of an image intensifier tube the envelope of which includes a glass body 21, a metal end cap 22 with an integral seal-off tubulation 23, and a cathode substrate or window 24 which passes the light or radiation to which the photocathode 25 is sensitive. The deposits 26 and 27 show two alternative positions of the decay-preventing layers which are topic of the present disclosure. Their presence on, practically, any place within the device reminds one of getter mirrors through their purpose is exactly to the contrary. Instead of binding desorbed traces of gas or vapor, they create a partial pressure of that constituent element of the cathode material which is most prone to evaporation. So far, incorporation of a flash mirror of yellow or red phosphorus, gray, yellow, or black arsenic, yellow or black antimony or bismuth is the most suitable method of stabilization, while the choice of the element is dependent on the constitution of the photoelectric cathode involved. Any of the said elements can be introduced into and spread within the device in a manner well known from established getter technology.

Obvious many variations of the abovestructures will occur to those skilled in the art, but the present invention is limited only as specified in the claims which follow.

I claim:

1. Ina stabilized image intensifier tube having an evacuated envelope andaphotoelectric cathode consisting chiefly of a compound of elements from the III- rd and V-th columns of the Periodic System, the improvement comprising:

at least one stain of a column V element coated on the inside of said envelope.

2. An image intensifier tube according to claim 1 wherein:

the selected stain element is chosen from a group consisting of yellow and red phosphorus.

3. An image intensifier tube according to claim 1 wherein:

the selected stain element is chosen from a group consisting of gray, yellow, and black arsenic.

4. An image intensifier tube according to claim 1 wherein the selected stain element is chosen from a group consisting of metallic, yellow and black antimony.

5. An image intensifier tube according to claim 1 where the selected stain element is bismuth.

'6. An image intensifier tube according to claim 1 wherein:

a plurality of stain elements are coated on said envelope chosen from a group consisting of phosphorus, arsenic, antimony and bismuth.

3 4 7. An image intensifier tube according to claim 1 wherein: wherein: the amount of material in at least one of said stains the amount of material m Sald 5mm Such that is such that the partial pressure of its vapor constittial pressure of its vapor constituents is less than the pressure of the saturated vapor at the operating 5 temperature of the device.

8. An image intensifier tube according to claim 6 uents is less than the pressure of the saturated vapor at the operating temperature of the device. 

1. IN A STABILIZED IMAGE INTENSIFIER TUBE HAVING AN EVACUATED ENVELOPE AND A PHOTOELECTRIC CATHODE CONSITING CHIEFLY OF A COMPOUND OF ELEMENTS FROM THE III-RD AND V-TH- COLUMUNS OF THE PERIODIC SYSTEM, THE IMPROVEMENT COMPRISING: AT LEAST ONE STAIN OF A COLUMN V ELENENTS COATED ON THE INSIDE OF SAID ENVELOPE.
 2. An image intensifier tube according to claim 1 wherein: the selected stain element is chosen from a group consisting of yellow and red phosphorus.
 3. An image intensifier tube according to claim 1 wherein: the selected stain element is chosen from a group consisting of gray, yellow, and black arsenic.
 4. An image intensifier tube according to claim 1 wherein the selected stain element is chosen from a group consisting of metallic, yellow and black antimony.
 5. An image intensifier tube according to claim 1 where the selected stain element is bismuth.
 6. An image intensifier tube according to claim 1 wherein: a plurality of stain elements are coated on said envelope chosen from a group consisting of phosphorus, arsenic, antimony and bismuth.
 7. An image intensifier tube according to claim 1 wherein: the amount of material in said stain is such that partial pressure of its vapor constituents is less than the pressure of the saturated vapor at the operating temperature of the device.
 8. An image intensifier tube according to claim 6 wherein: the amount of material in at least one of said stains is such that the partial pressure of its vapor constituents is less than the pressure of the saturated vapor at the operating temperature of the device. 